After checking the service manual I found out that my previous post is not correct.
The headphones and speakers have separate amplifiers. Also woofer and both L+R speakers (midrange and tweeter) have separate power amplifiers. You say that the crackle is only in the left speaker (mid range and tweeter). If it is not in the woofer then you will probably not hear it In the headphone either, also not in the line output. Woofer and other speakers partly use same preamplifier (equalizer). This means that the source of he crackle probably will be in the power amplifier of the left speakers or in the speakers itself. You should check the presence of the crackle in all available outputs (sub-out, line out, headphone, woofer, midrange sp and tweeter) to find the source of the crackle. Good luck!
Bert